1. Field of the Invention
The present invention relates generally to the field of immunology and in particular, vaccinology. It concerns the identification of immunotherapeutic/immunogenic peptoids and the development of peptoid vaccines for the prevention and treatment of disease.
2. Description of Related Art
According to the Centers for Disease Control, there are over 40 vaccines currently approved for use in the United States. In all cases, these vaccines work by inducing protective immunity, i.e., generating antibodies that can prevent or limit the infection/damage by organisms or toxins that breach epithelial or mucosal barriers, or that can neutralize toxins. In addition to inducing and maintaining long lasting circulating antibody, vaccination schedules are designed to maintain pools of memory lymphocytes that are “on call” for a rapid response to the pathogen or toxin years or even decades later, inducing the production of immunoglobulin (Ig) IgG, IgE, or IgA in the blood or mucosal surfaces, respectively. While these recall responses take a few days to appear, they are often sufficient to abort serious infections. For toxins, which can kill an individual very quickly, antibodies must already be on board in the blood. Cytotoxic T cells are important in resolving intracellular infection, but as far as is known, no approved vaccines work solely by this mechanism; the production of antibody is the major effector mechanism and means of conveying protective and/or sterilizing immunity.
In order to generate a robust IgG, IgE, or IgA response, a molecule must be immunogenic and T cell dependent Immunogenicity is determined by the presence of structural determinants or epitopes on the molecule that can be recognized by two different lineages of lymphocytes, B cells and T cells. Once the B cell epitope on the immunogen is bound to a receptor on a clone of B cells, it is internalized, degraded and peptides from the degraded protein are recycled to the cell surface in human leukocyte antigen (HLA) molecules. These HLA-presented peptides are T cell epitopes. They can be presented on dendritic cells and macrophages as well as B cells. Once the T cells recognize the HLA-peptide on the dendritic cell, and the B cell that initially recognized the native antigen, the helper T cell induces the B cell to differentiate into plasma cells that make antibodies against the B cell epitope (seen by the B cells on the native molecule). Progeny of the helper T cell also help the B cells make IgG, IgE, or IgA of high affinity. If either T cell epitopes or B cell epitopes are lacking on a given molecule, there will rarely be an IgG, IgE or IgA antibody response. However, in the absence of T cell help, some B cell epitopes linked to mitogens or nanoparticles can induce IgM responses.
Several different types of vaccines have been approved for human use. These include live attenuated pathogens, dead pathogens, extracts of pathogens, proteins, subunits, or carbohydrates from pathogens, inactivated toxins, or recombinant proteins. Live attenuated vaccines include polio, mumps, measles, rubella, smallpox, chicken pox and influenza. In general the viruses are grown in non-human cells (such as chicken eggs or simian cells) until they have mutated sufficiently to grow for a limited time in humans but not cause disease. In some cases, mutations are intentionally introduced into the genome of the wild type pathogen to prevent it from causing disease. In other cases, a cross-reactive non-human pathogen is used. In general, these vaccines cause transient infections in a tissue site such as the gut, lung, nasopharynx or skin. Because they grow for a period of time, they induce robust immunity against a variety of B cell epitopes and the antibody responses are long lived. The long lived antibody response is due to long-lived plasma cells and the activation of large pools of memory B and T cells. If given orally or intranasally, live attenuated pathogens can sometimes induce an IgA response in the mucosa which is thought to prevent pathogens from breaching mucosal surfaces in the lung, gastrointestinal and urogenital tracts. The vaccine-induced infection is self-limited by the immune response induced against it. In making these vaccines from live attenuated pathogens, there are several issues that must be considered. First, the pathogen must be sufficiently altered so that it cannot back-mutate and cause disease. Secondly, if individuals are allergic to the cells or components of the cells in which the intracellular pathogens or viruses are grown, they are not eligible to receive the vaccine. Third, (albeit rarely) the vaccines carry the risk of transmitting oncogenic viruses from the non-human cells in which they are grown. Fourth, the immunodominant and protective naturally-expressed epitopes can undergo mutation either within the individual or within the strain of virus from year to year such that frequent vaccinations are necessary and sometimes antibody-resistant organisms emerge. Finally, immunocompromised individuals are at risk for infection because the live organism cannot be cleared or it can survive long enough to revert to the wild-type organism. In general, these vaccines are expensive to make and in some cases there is enough hype about their side effects that individuals refuse to be vaccinated. In certain populations, these infections have therefore reappeared (e.g., Polio and Pertussis).
Dead pathogen vaccines are generally injected intramuscularly with a strong adjuvant to induce the appearance of IgG antibodies in the blood and tissues. It is difficult, but not impossible to induce mucosal immunity with such vaccines, although there are several strategies under study to circumvent this difficulty. If they are given by injection into the muscle or dermis, they can induce a systemic (blood/tissues) antibody response and prevent pathogens or their products from traveling from the site of infection into the bloodstream to another target organ. These vaccines are also expensive to make, and in the case of some, i.e., influenza, the most immunodominant natural epitopes mutate from year to year so that a new vaccine must be manufactured and given annually based on the best prediction of what strain of virus will infect the American public. Sometimes the predictions for the annual strain that will infect the American public are wrong and even if they are correct, annual immunizations are required. In addition, such vaccines can be problematic in the young or elderly where primary immune responses must be made each year and in up to 40% of such individuals, they are not. That is because the young and the elderly often have suboptimal immune systems (except for recall responses in the elderly). It is probably the case that a proportion of individuals in their prime years do not make good antibody responses either, but this is still under study. Compliance is an additional issue. In the case of the human immunodeficiency virus (HIV) or Hepatitis C virus (HCV), dead vaccines would only protect against one or a limited number of subtypes of the virus, since different subtypes have different immunodominant antigens.
Recombinant proteins or subunits vaccines require prior knowledge of the immunoprotective epitopes; they must have both B and T cell epitopes and induce a robust immune response, which is often difficult in the absence of infection or tissue damage. Adjuvants are virtually always necessary to get a robust long-lived response. Obviously, these vaccines must also contain epitopes that are conserved among different strains/clades/or subtypes of the pathogen.
Conjugate vaccines consist of a B cell epitope linked to a carrier protein that contains T cell epitopes. At present, the B cell epitope is generally a carbohydrate to which a young child cannot respond and to which an adult will make only an IgM antibody that does not affinity-mature (i.e., get better with boosting). The selection of the B cell epitope also requires knowledge of the immunogenic carbohydrate or other structure on the pathogen that will elicit neutralizing antibodies. Such vaccines can be effective but they are expensive.
Other vaccines under development include peptides, pathogen genomes packaged in viruses or plasmids, dendritic cell vaccines and anti-idiotypic vaccines. With regard to peptides, these are usually aimed at inducing T cell responses and not antibody responses although there are exceptions. The correct peptides will bind to HLA on antigen presenting cells (APCs) and prime T cells so that they can kill cells infected with intracellular non-lytic pathogens such as HIV or HCV. To design these vaccines the peptide must be of the correct size and have anchoring motifs that bind to the HLA antigens of most of the human species and induce a protective T cell response. Most peptides of this nature are not designed to contain B cell epitopes. It is unknown at this time whether these vaccines will have any utility in humans to induce protective or sterilizing immunity.
In sum, all these vaccines require either inactivated pathogens or extracts thereof or biologically attenuated toxins (called toxoids) or prior knowledge of the immunogen that will induce protective antibody. That immunogen must contain both T and B cell epitopes if production of class-switched IgG or IgA antibody is the goal. All are also expensive to make and several have side effects. Even existing vaccines would benefit from new designs that would make them safer, cheaper, more immunogenic and able to circumvent the problem of genetic drift or mutation of the immunizing epitopes. In addition, there are many pathogens and toxins against which there currently are no effective and/or approved vaccines. These pathogens take a major toll on humans in both developed countries and especially in the third world. Other pathogens and toxins are of concern in this era of bioterrorism. Pathogens not typically endemic in the U.S. can also be a threat to travelers abroad. In our mobile society, emerging infection pathogens such as severe acute respiratory syndrome (SARS) or Ebola virus can be transported around the world in a matter of days. Thus, improved methods of identifying vaccine antigens are urgently needed.